1. Field of the Invention
The present invention relates to a multilayer board and, more particularly, to a flexible multilayer board.
2. Description of the Related Art
Because of their utility in limited spaces, flexible boards have been widely used in electronic devices, such as cameras, mobile phones, personal computers, printers, and hard disk drives. With the reduction in size of electronic devices, flexible boards are becoming smaller, higher in density, and more multilayered.
One such flexible board is a flexible board (flexible print circuit board) that includes thermoplastic polyimide films and copper foil (conductor layers) alternately stacked on top of one another (see, for example, paragraph [0014] in Japanese Unexamined Patent Application Publication No. 7-245460).
As described above, flexible boards are suitably packaged in limited spaces and can be bent many times. Thus, flexible boards are suitably used in electronic devices that will inevitably be deformed during use.
As flexible boards become smaller and higher density, and as conductor layers (such as internal conductor patterns) become thinner, the likelihood of a disconnection occurring in a conductor layer, such as an internal conductor pattern, increases under repeated stress due to deformation, such as bending.
In the manufacture of a flexible board by stacking and pressing thermoplastic resin layers and conductor layers, such as internal conductor patterns, variations in the density of an internal conductor pattern cause variations in pressing force during the multilayering process, and a portion of the conductor layers, such as internal conductor patterns, that is subjected to increased force is often disconnected.
Thus, there is a demand for a reliable flexible board in which disconnection does not occur in a conductor layer, such as an internal conductor pattern, in a stacking and pressing process or in use of a product that repeatedly causes deformation of the flexible board.
A flexible board described in Japanese Unexamined Patent Application Publication No. 7-245460 is, for example, drawn at a predetermined temperature (for example, 300° C.) to form a flexible printed circuit board having a desired shape (see paragraph [0015] in Japanese Unexamined Patent Application Publication No. 7-245460) and cannot be used in applications that repeatedly cause deformation of the flexible board.